1. Field of the Invention
The present invention generally relates to an anti-lock brake control system for controlling braking hydraulic pressure for brake application to a plurality of wheels of a motor vehicle on a wheel-by-wheel basis. More particularly, the invention is concerned with an anti-lock brake control system for arithmetically determining a pseudo vehicle speed on the basis of wheel rotation speeds corrected in consideration of operating states of the individual wheels.
2. Description of Related Art
As a hitherto known system for arithmetically determining a pseudo vehicle speed, there may be mentioned, by way of example, a slip control system for a motor vehicle which is disclosed in Japanese Unexamined Patent Application Publication No. 329009/1994 (JP-A-6-329009) and which includes a wheel speed detecting means for detecting wheel speeds of individual wheels, a pseudo vehicle speed arithmetic means for calculating a pseudo vehicle speed on the basis of the wheel speeds detected by the wheel speed detecting means, and a brake application pressure regulating means for decreasing or lowering the brake application pressure applied to the wheel when the rotation speed thereof falls within a predetermined slip or skid range (indicating locked state of the wheel), to thereby prevent the wheel from being locked and allow the stopping distance to be shortened to a possible minimum.
For having better understanding of the invention, the background art thereof will be reviewed in some detail by referring to FIG. 18 of the accompanying drawings which shows in a flow chart a conventional method of arithmetically determining a pseudo vehicle speed. Referring to the figure, wheel speeds Vfr, Vfl, Vrr and Vrl of a front right wheel, front left wheel, rear right wheel and a rear left wheel, respectively, of a motor vehicle are arithmetically determined by wheel speed detecting devices (not shown) in a step S91 Subsequently, in a step S92, a first pseudo vehicle speed Vr1 is arithmetically determined on the basis of a higher one of the wheel speeds of the front left wheel and the rear left wheel In a succeeding step S93, a second pseudo vehicle speed Vr2 is calculated on the basis of a higher one of the wheel speeds of the front right wheel and the rear right wheel. Additionally, a third pseudo vehicle speed Vr3 is determined on the basis of the highest one of the wheel speeds of all the four wheels in a step S94
Subsequently, the third pseudo vehicle speed Vr3 is corrected by using a value obtained by subtracting a correcting quantity k (k&gt;0) from the third pseudo vehicle speed Vr3 in a step 95, where the correcting value k represents a value which corresponds to a difference in the rotation speed between the wheels located at inner and outer sides, respectively, as viewed in a turning direction when the motor vehicle is turned. In a step S96, the first pseudo vehicle speed Vr1 and the third pseudo vehicle speed Vr3 corrected are compared with each other to thereby determine the higher pseudo vehicle speed as the left-side pseudo vehicle speed Vrfl.
Similarly, the second pseudo vehicle speed Vr2 is compared with the third pseudo vehicle speed Vr3 corrected in the manner mentioned above, to thereby determine the higher pseudo vehicle speed as the right-side pseudo vehicle speed Vrfr. On the basis of the two pseudo vehicle speeds Vrfl and Vrfr as obtained, magnitudes of slips of the two wheels located at the left side of the motor vehicle, as viewed forwardly in the moving direction of the motor vehicle, are arithmetically determined on the basis of the left-side pseudo vehicle speed and the relevant wheel speeds. Similarly, magnitudes of slips of the two wheels located at the right side are calculated on the basis of the right-side pseudo vehicle speed Vrfr and the relevant wheel speeds.
As is apparent from the above description, according to the pseudo vehicle speed calculating method adopted in the conventional slip control apparatus for the motor vehicle known heretofore, the right-side pseudo vehicle speed is so selected or determined as to be at least equal to or higher than the higher one of the wheel speeds of the right-side front and rear wheels. Similarly, the left-side pseudo vehicle speed is so selected as to be higher than or at least equal to the higher one of the wheel speeds of the left-side front and rear wheels.
Under the circumstances, in the case of a motor vehicle in which the driving wheels are operatively coupled to an internal combustion engine by means of a drive shaft so that driving torque is transmitted to each driving wheel from the engine, there may occur such situation that the wheel speed becomes higher than the actual vehicle speed. Furthermore, when the rotation speed of one of the right and left driving wheels operatively coupled to each other by means of a differential mechanism such as a differential gear decreases or sinks steeply, then the rotation speed of the other wheel hops or increases steeply, which may possibly result in the wheel speed increasing beyond the vehicle speed.
For the reasons mentioned above, the pseudo vehicle speed may also increase beyond the actual vehicle speed when the speed of the first wheel increases or rises steeply. Consequently, when the pseudo vehicle speed is arithmetically determined on the basis of the steeply increased wheel speed, the pseudo vehicle speed as calculated increases steeply as well. Additionally, the pseudo vehicle speed determined for the wheel located at the same side or in a same row as the wheel whose speed increase steeply will equally increases. Consequently, when the slip is determined on the basis of the pseudo vehicle speed mentioned above, magnitude of the slip or skid as determined tends to become greater than the actual slip (skid).
Furthermore, when a steering handle of the motor vehicle is rotated over a wide angular range at a high rate while applying the brake upon turning of the motor vehicle, there may arise difference in the rotation speed between the front wheel and the rear wheel which are located at the outer side as viewed in the turning direction of the motor vehicle. Besides, because the pseudo vehicle speed is the same for both the front wheel and the rear wheel when the motor vehicle runs at a speed equal to the pseudo vehicle speed, magnitude of the slip as calculated becomes greater than that of the slip actually taking place.
As will now be appreciated from the above, when the pseudo vehicle speed is higher than the wheel speed of the individual wheels and thus when the magnitude of the slip or skid which is equivalent to the difference between the pseudo vehicle speed as calculated and the wheel speed is greater than that of the slip or skid actually occurring, the braking hydraulic pressure tends to be reduced excessively upon brake operation in an effort to prevent the slip from becoming large, which in turn means that the brake performance of the motor vehicle is degraded, giving rise to a problem that the stopping distance of the motor vehicle increases undesirably